The present invention relates to a shaft member for fluid lubrication bearing apparatuses which relatively rotatably supports a shaft member in the radial direction by a lubricating film of a fluid which occurs in a radial bearing gap and a method for producing the same.
Fluid lubrication bearings of this type are roughly classified into two groups: a hydrodynamic bearing comprising a hydrodynamic pressure producing means for producing hydrodynamic pressure in a lubricating oil in a bearing gap; and so-called cylindrical bearings (bearings comprising a perfectly circular bearing face) not comprising a hydrodynamic pressure producing means.
For example, a fluid lubrication bearing apparatus incorporated in a spindle motor of a disk drive unit such as HDD is provided with a radial bearing portion which rotatably supports a shaft member in the radial direction in a non-contact manner; and a thrust bearing portion which rotatably supports the shaft member in the thrust direction in a non-contact manner. a bearing (hydrodynamic bearing) which is provided with grooves for producing a hydrodynamic pressure (hydrodynamic groove) on the inner periphery face of a bearing sleeve or on the outer circumferential surface of the shaft member is used as a radial bearing portion. As a thrust bearing portion, for example, both end faces of the flange portion of the shaft member or a hydrodynamic bearing provided with hydrodynamic grooves on the face facing it (an end face of the bearing sleeve, an end face of a bottom member fixed on a housing, the inner bottom face of the bottom of the housing or the like) is used (for example, refer to patent document 1: Japanese Unexamined Patent Publication No. 2002-61641). Alternatively, as the thrust bearing portion, a bearing having the structure in which one end face of the shaft member is supported in a contact manner by a bottom member (so-called pivot bearing) is used in some cases (for example, refer to patent document 2: Japanese Unexamined Patent Publication No. 1999-191943).
In a spindle motor of this type, a clamper for clamping a disk-shaped information recording medium such as magnetic disks (hereinafter referred to simply as a disk) between a disk hub and itself is attached to the edge of the shaft member. The clamper is attached on the shaft member by screwing into a threaded hole formed on one edge of the shaft member through the clamper (for example, refer to patent document 3: Japanese Unexamined Patent Publication No. 2000-235766).
Recently, to deal with increased information recording density and higher rotational speed in information appliances, higher rotational accuracy is required for the above spindle motor for information appliances. To meet this demand, higher rotational accuracy is also required for a fluid lubrication bearing apparatus incorporated into the above spindle motor. At the same time, with the demand for lower price of information appliances, reduced production costs of the above fluid lubrication bearing apparatus are strongly desired recently.
In order to stably provide the rotational performance of a fluid lubrication bearing apparatus (hydrodynamic bearing apparatus) for the long term, it is important to control the radial bearing gap and thrust bearing gap, in which the pressure of the fluid for supporting the shaft member is present, to be highly accurate. For example, when the thrust bearing gap is formed on both side of the flange portion in the axial direction as mentioned above, to maintain the thrust support of the shaft member in a stable state, the pressure for the thrust support on one end face side of the flange portion and the pressure for thrust support on the other end face side need to be brought into balance so that the sliding contact of the end face of the flange portion and the face facing it is avoided as much as possible. Higher accuracy of the thrust bearing gap can be achieved by processing the end face of the flange portion facing this, hydrodynamic grooves and the like highly accurately, but merely increasing processing accuracy will inappropriately result in higher costs.
Moreover, examples of methods for forming a threaded hole on the shaft member include a method comprising forming a prepared hole of the threaded hole on the shaft material by cutting, and a thread cutting is worked relative to this prepared hole. However, by this method, cutting powders produced when the prepared hole is cut are accumulated at the bottom of the threaded hole, and cutting powders cannot be completely removed even if the threaded hole is cleaned after the process. Accordingly, the cutting powders remaining inside the threaded hole deposit to other components as contaminants when other components are mounted or a bearing apparatus is assembled, and may get in the fluid (for example, lubricating oil, etc.) filling the inside of the bearing apparatus after being assembled. Alternatively, if the cutting powders deposited to other components (contamination) are further transferred to disks, they may cause disk crash. Moreover, removal of cutting powders requires complicated and numerous cleaning steps, leading to an increase in costs.
Moreover, to deal with cost reduction required for the above fluid lubrication bearing apparatus, various cost reducing measures are examined for the component parts of fluid lubrication bearing apparatuses. For example, as for the shaft member, an article comprising the shaft portion and the flange portion integrally formed by forging to produce a near net shape is known (for example, refer to patent document 4: Japanese Unexamined Patent Publication No. 2004-347126).